1. Field of the Invention
This invention relates generally to rectifier transformers used with rectifier assemblies and other power conversion apparatus for transforming an alternating current signal to direct current, and more specifically to a rectifier transformer circuit for single way, six pulse operation, 120.degree. diode and secondary winding conduction, with an integral phase-shift winding arrangement, and without an interphase transformer.
2. Description of the Prior Art
Several transformer configurations for use with semiconductor power rectifiers are well-known in the art. The transformer primary winding is connected to a three-phase ac power line to provide voltage to the rectifiers, via a secondary winding, connected to furnish voltages at uniformly distributed phase angles. Since rectifiers tend to draw a square wave current from the power system, they are a source of harmonic currents in the power system. Such harmonic currents induce noise voltage in adjacent communication circuits and cause additional and unwanted heating in motors, capacitors, and generators.
To reduce the harmonics, it is common practice in rectifier installations to increase the number of rectifier phases (i.e., the number of pulses appearing in the rectified dc signal). For large rectifier installations, 96-phase operation is not uncommon. To obtain the necessary multi-phase operation it is customary to use six pulse circuits with phase-shifted rectifier transformers having delta or wye primary windings. The secondary windings and rectifiers for each transformer are identical. As an example, a typical 24-phase rectifier system consists of four rectifier transformers each arranged for six-pulse operation and connected as follows: (1) wye plus 71/2.degree. shift, (2) wye minus 71/2.degree. shift, (3) delta plus 71/2.degree. shift, and (4) delta minus 71/2.degree. shift. This arrangement provides 24-phase overall operation, and theoretically the ac side has harmonics of only multiples of twenty-four plus or minus one. It is apparent that other combinations of phase shift are possible to obtain a 24-phase system.
To provide this multi-phase operation the prior art rectifier circuits require, in one type of rectifier assembly a separate phase-shift transformer, and in a second type of rectifier assembly a phase-shift winding as a part of the rectifier transformer and an interphase transformer. An advantage of the present invention is the elimination of the phase-shift transformer in the first design, and elimination of the interphase transformer in the second.